Add cross-platform custom decorations (titlebar-less) + platform titlebar demo + small fixes

[JakkuSakura]

This PR improves the “custom titlebar / custom decorations” story end-to-end: it adds two examples, fills in a missing backend capability (X11 window menu), and fixes a macOS
crash when toggling decorations.

What’s in this PR

• New example: winit/examples/custom_decorations.rs
  • Demonstrates titlebar-less windows via WindowAttributes::with_decorations(false).
  • Shows the minimal primitives needed to keep expected window UX when drawing your own chrome:
    • Move: Window::drag_window() by dragging a top bar region.
    • Resize: Window::drag_resize_window(ResizeDirection) by dragging edges/corners.
    • Window/system menu: Window::show_window_menu(position) on right click (no-op where unsupported).
    • Basic caption buttons (minimize/maximize/close) with hover hit-testing and cursor feedback.
  • Rendering is intentionally toolkit-free: solid background + top bar + button rects using softbuffer.
• New example: winit/examples/custom_titlebar.rs
  • Demonstrates “OS-provided titlebar customization knobs” where available:
    • macOS: transparent/unified titlebar and related attributes.
    • Windows: titlebar/border colors and backdrop type.
  • Includes runtime toggles for quick manual verification.
• Example support utilities: winit/examples/util/fill.rs
  • Adds fill_window_with_top_bar(...) and fill_window_with_top_bar_and_rects(...) (plus a tiny Rect helper) so examples can render simple custom chrome without a UI toolkit.

Backend/library changes

• X11: implement Window::show_window_menu via _GTK_SHOW_WINDOW_MENU when the WM advertises support.
  • winit-x11/src/atoms.rs
  • winit-x11/src/window.rs
• Docs: update platform support notes for Window::show_window_menu.
  • winit-core/src/window.rs
• macOS: avoid panic when the view is temporarily detached from a window (can happen during decoration toggles).
  • winit-appkit/src/view.rs
  • (plus small related adjustments in AppKit crate as needed)

Why this is necessary

• decorations(false) is the foundation for titlebar-less windows across platforms, but without a canonical recipe users frequently ship windows that can’t be moved/resized or that
  lose access to the system menu.
• The new example provides a minimal, portable blueprint for “draw your own titlebar” using existing winit primitives, and acts as a regression target for pointer events + drag/
  window-menu APIs.
• X11 window menu support closes a real gap in the custom-decorations story (Windows and Wayland already support it; X11 now does where the WM cooperates).
• The macOS detachment fix addresses a crash encountered when toggling decorations rapidly.

How to test

• cargo run -p winit --example custom_decorations
  • Drag top bar to move; drag edges/corners to resize; right click to request system menu; click caption buttons.
• cargo run -p winit --example custom_titlebar
  • Use keybindings to toggle platform-specific titlebar options.

Notes / Limitations

• X11 window menu is best-effort: it depends on WM support for _GTK_SHOW_WINDOW_MENU (checked via _NET_SUPPORTED).
• Caption button visuals are intentionally simple; example focuses on mechanics and API usage rather than native styling.

closes #3664
related to #4100, but my focus in on MacOS and adds examples. I'm fine merging the code and test if either is merged first